1. Field of the Invention
The present invention relates to a switch mode power supply (SMPS). More particularly, the present invention relates to a quasi-resonant switching type SMPS and a driving method thereof.
2. Description of the Related Art
An SMPS is a device for rectifying an input AC voltage into an input DC voltage (dc-link voltage) and converting the input DC voltage into a DC output voltage having a different level. The DC output voltage can be greater than or equal to the input DC voltage. The SMPS is generally used for battery power supplies that power electronic devices, and particularly, mobile terminals or laptop computers.
The quasi-resonant switching mode based SMPS includes a quasi-resonant converter for supplying power according to switching by a main switch.
FIG. 1 shows a relationship between the output power of a conventional quasi-resonant SMPS and a switching frequency of the main switch.
As shown in FIG. 1, the switching frequency decreases as the output power of the SMPS increases. In detail, the output power depends on the load connected to the SMPS. When the load is light, the peak of the current flowing to the drain of the main switch is reduced and the switching frequency is increased, which is shown by the dotted line (a) of FIG. 1. The increase of the switching frequency under light load condition generates a switching loss. To prevent this, the conventional quasi-resonant converter restricts the switching frequency for light loads at some predetermined value, approximately independent from the output power, as shown in FIG. 1(b).
However, when the switching frequency of the main switch is restricted to a predetermined value, it is a challenge to perform switching at the lowest switch voltages. This non-optimal operation at low switching voltages is disadvantageous and leads to a switching loss. This switching loss is also enhanced in the case of a large input voltage. Therefore, for low loads connected to an output terminal of the SMPS, it would be desirable to increase the efficiency of the burst mode switching operation.
The burst mode switching operation represents a repeated switching operation that is stopped for a predetermined time after the SMPS outputs the power.
FIG. 2 shows a burst switching reference period when the load of the quasi-resonant SMPS is increased. When a power device performs a burst mode operation, the reference frequency is typically below 2-3 kHz, which is the frequency audible to a person, so that the person can hear no audible noise caused by the burst switching. However, as the load current is gradually increased, the interval, in which no switching is performed during the burst mode operation, is gradually reduced changing the burst mode into the normal mode. In this normal mode operation of the SMPS the frequency of the noise may move into the 2-3 kHz frequency range, which is audible to the user, which is undesirable.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.